A digital key telephone system comprises a main unit connected to a public network or a private branch exchange and a plurality of digital key telephones connected to the extension lines of the main unit. The digital key telephone has a plurality of push button type keys. When these keys are depressed, calling the public network or the private branch exchange, answering a call, and speech communication between digital key telephones are allowed. The main unit processes not only a control signal but also a speech communication signal as a digital signal and executes calling processing/call reception processing of each digital key telephone by a stored program control scheme and switching processing by a time switch of the digital speech communication signal.
As one of analog communication networks provided in U.S.A. recently, there is a communication network for providing a service called, e.g., "CLASS (Custom Local Area Signaling Service)". The communication network for providing the CLASS service can superpose calling line identification information on a call signal and transmit it to the called subscriber.
When the digital key telephone system is connected to such a communication network, the main unit of the called digital key telephone system detects the first bell signal and then a modem signal. When calling line identification information is detected from the modem signal, the calling line identification information is transferred from the main unit to the called key telephone and displayed on the liquid crystal display device (LCD) of the key telephone. The user of the key telephone can recognize the calling line before off-hook by checking the calling line identification information displayed on the liquid crystal display device.
Conventionally, however, as the main function of the digital key telephone system using the calling line information notification service in the analog communication network such as the CLASS service, only the above-described calling line identification information display function is realized. Therefore, a demand has arisen for realization of a new and effective functional service using the CLASS service.
Another demand has arisen for expansion or realization of various support functions associated with wiring installation, a function of allowing a variety of call reception processing by adding a plurality of extension numbers to the digital key telephones, and a function of allowing a variety of communication processing operations using a personal computer combined with the digital key telephones.